waiting in vain for an unlikely event
A yojijukugo literally meaning 'waiting a hundred years for the Yellow River to clear', used to describe futile waiting for something that will never happen. Equivalent to English idioms like 'when pigs fly'.
Waiting for him to keep his promise is like waiting for pigs to fly.
From the Chinese saying that one may wait for the perpetually muddy Yellow River to run clear, while human life is too short for such an event. 百年河清 consequently denotes waiting in vain for something exceedingly unlikely.